Suture Materials and Needles
Danny R. Fijalkowski
Sutures and needles are an integral part of surgery. There is a wide array of suture and needle products available, many designed with specific surgical applications, such as vascular, orthopaedic, and ophthalmic, in mind. Differences in the physical construction and chemical composition of suture and needle products endow each with attributes that make it advantageous or disadvantageous for different situations. Using the most appropriate suture and needle will have a positive effect on the surgical outcome and surgeon and patient satisfaction. Selecting the best suture and needle for a particular surgical task requires a familiarity with the available products. The goal of this chapter is to offer the reader a background and an overview of sutures and needles commonly used by the podiatric surgeon.
SUTURES
In basic terms, a suture is a strand of material that can be used to bring soft tissues into apposition and to tie off or ligate blood vessels. Once implanted, sutures must provide strength at the wound margins during a critical period of wound healing, when tissue lacks sufficient strength to support itself (1,2). Until sufficient wound healing has taken place, the burden of all forces acting at the wound margin is placed solely on the suture. If the suture fails to maintain apposition of the wound margins, further soft tissue damage, loss of correction, or wound dehiscence may result.
The history of surgical suture goes back thousands of years. The earliest known written accounts of wound closure with sutures and needles are evidenced in ancient Egyptian scrolls dated 3500 BC (3,4). The earliest materials were fashioned from animal sinews, animal hair, and plant fibers (3). Today, we use a variety of suture materials, natural and synthetic, absorbable and nonabsorbable, with single and multifilament strand configurations. They may be uncoated or coated, naturally colored or colored with FDA-approved dyes. Variables in the composition and construction of a suture affect everything from the ease with which the product is handled to its interaction and performance once inserted into the body.
The United States Pharmacopeia (USP) is the official compendium for defining and describing the physical characteristics of each suture material (1,5). These characteristics include strength, size, needle attachments, and ability of the material to absorb fluid (1,5). The USP also sets forth standards for the manufacturing, packaging, storage, sterilization, and labeling of sutures (1,5).
Sutures are sterilized with either ethylene oxide gas or ionizing radiation, usually from cobalt 60 (1,6). Both are considered cold sterilization because exposure to heat, as with an autoclave, may cause damage to the product or its packaging (1,6,7 and 8). Some suture materials, such as cotton, must be sterilized with ethylene oxide gas, as ionizing radiation may damage its fibers (6,7).
Suture packaging is designed to protect the material from environmental factors such as light, oxygen, and moisture. Package design maintains sterility of the product and allows for an easy means of suture delivery to the sterile field of operation. Each package is labeled with information regarding the enclosed product to include the material from which the suture is made; whether it is coated or uncoated; suture type, either natural or synthetic, absorbable or nonabsorbable; its size, both metric and USP size designation; suture length; suture configuration; description of the needle; the number of sutures included; lot number; name of the manufacturer; trade name of the product; and expiration date (5). Most sutures are packaged dry; however, collagen-based sutures are packaged wet, usually in alcohol, to prevent them from drying out (7). “Pliabilized” nylon suture (Ethicon, Somerville, NJ), used for cosmetic and plastic surgery, is packaged premoistened, which makes it easier to handle (6).
The USP divides sutures into two categories: absorbable and nonabsorbable (5). These main categories can then be subdivided into natural and synthetic materials for each. An absorbable suture is one that will lose most of its strength within 60 days following implantation (1). These sutures include catgut, polydioxanone (PDS), polyglactin 910, and polyglycolic acid (PGA) (1). Absorbable sutures may be natural, of animal origin (collagen), or synthetic (1) (Table 4.1). These materials may be fashioned into monofilament or multifilament strands to offer the best handling and performance characteristics for that product. They may be of natural color or dyed to maximize their visibility. Absorption of the suture means that its presence in the soft tissues is temporary, as is its capacity to hold tissues together. The initial strength of an absorbable suture material is directly proportional to its thread diameter. The rate at which it loses its strength, however, is independent of its size and rate of absorption (9). Absorption begins as a loss of tensile strength followed by a loss of mass of the implanted product. Some materials are present in the tissues long after they have lost their effective tensile strength. Therefore, familiarity of each absorbable sutures interaction with the body, its rate of absorption, and the rate at which it loses its strength must be weighed against the anticipated healing time for the tissues being sutured (2).
The term nonabsorbable is a relative term because many of the sutures that we consider to be nonabsorbable are in fact degraded in the tissue, although at a very slow rate (1) (Table 4.2). Some sutures are not subject to absorption no matter how long they have been implanted. These include stainless steel, polyester, polybutester, and polypropylene (1). Nylon and silk, although classified as nonabsorbable, are absorbed over many months (1,10). Soft tissue reaction to nonabsorbable sutures is, overall, milder than that of the absorbable sutures (Table 4.3). Nonabsorbable sutures may be coated or uncoated, multifilament or monofilament to maximize their handling properties. They may be uncolored or dyed to offer better visibility (6).
TABLE 4.1 Absorbable Sutures | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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SUTURE CONFIGURATION
Sutures may be described and differentiated in terms of their fiber arrangement. The physical configuration of a suture refers to whether it is fashioned as a monofilament or a multifilament structure (1) (Tables 4.1 and 4.2). A suture’s configuration will affect its handling characteristics and its interaction with the body.
Monofilament sutures are a single strand of either an absorbable or nonabsorbable material. The monofilament configuration provides an overall smoother, low-friction surface that allows the suture to glide easily through soft tissues and slip easily, but not necessarily securely, into a knot. Another benefit of this smooth suture surface is that it provides less shelter for bacteria.
Many of the synthetic absorbable suture materials are very stiff, which precludes their use as a monofilament suture (11). Instead, thin strands of these materials are fashioned into multifilament strands, which affords them greater flexibility and strength (11). Some newer synthetic materials such as PDS and polyglyconate, however, offer excellent handling characteristics and strength, even as a monofilament.
Multifilament suture materials can be natural or synthetic, absorbable or nonabsorbable. They are formed by twisting or braiding several single filaments together to form one suture strand of a desired thickness. Synthetic multifilament sutures are formed by first liquefying the polymer and then extruding it through spinnerets to form narrow, monofilament strands. These strands are then braided or twisted together to form an appropriate-size multifilament suture thread (1,12). Multifilament synthetic sutures are relatively flexible and easy to handle as compared with monofilament synthetic sutures that tend to be stiff and wiry. Uncoated multifilament suture has a rough texture that generates high levels of friction when moved through soft tissue. High levels of friction create soft tissue drag that can cause tissue damage and make insertion and removal of suture difficult. A beneficial consequence of this rough, high-friction surface is that it affords a relatively secure knot. Uncoated multifilament sutures also have a tendency to absorb and harbor bacteria in their fiber arrangements. It is for these reasons that many multifilament sutures are offered with coatings that help to reduce tissue drag and lower the propensity for bacterial uptake between suture filaments.
SUTURE PROPERTIES
The ideal suture material would be strong and easy to handle, tie securely into a knot, and have no adverse effect on tissues (8). It would perform reliably and predictably, maintaining adequate strength across wound margins during critical times in wound healing until it was either removed or absorbed (8). There is no one ideal suture for every job; therefore, decisions must be made to choose a suture with the attributes necessary to achieve the best surgical outcome with the least compromise from that of the ideal. A variety of materials and thread configurations are available that give sutures their unique biologic and mechanical properties. Variations in these properties will facilitate comparisons among suture materials and offer surgeons more options.
MECHANICAL PROPERTIES
Mechanical properties of a suture describe its physical behavior in terms of handling characteristics, its form, and how it functions once implanted (2).
Tensile Strength
Tensile strength is defined as the amount of weight required to break a suture divided by the suture’s cross-sectional area (1). Tensile strength will vary among suture materials (Table 4.4). Stainless steel has the highest followed by the synthetic materials
with natural materials having the least (1). A suture’s effective tensile strength is its tensile strength when looped and knotted (1). Although variation occurs among suture materials and with different knot types, a knotted suture may retain only one-third of the tensile strength of the unknotted suture (1,13,14).
with natural materials having the least (1). A suture’s effective tensile strength is its tensile strength when looped and knotted (1). Although variation occurs among suture materials and with different knot types, a knotted suture may retain only one-third of the tensile strength of the unknotted suture (1,13,14).
TABLE 4.2 Nonabsorbable Sutures | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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TABLE 4.3 Relative Soft Tissue Reaction among Suture Types | ||||||||
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Suture diameter or caliber is a measure, in millimeters, of its cross-sectional diameter (1). A suture’s size designation is determined by guidelines set forth by the USP (1,5). Originally, sutures were given size designations no. 1 to no. 6,
with no. 1 being the smallest diameter. Newer materials and manufacturing techniques then allowed for the creation of smaller-diameter sutures. First, size 0 was accomplished; thereafter, more 0’s were tacked on to the size designation to indicate ever decreasing size diameters. The smaller the suture diameter, the more 0’s are added to the size designation. For example, a 5-0, or 00000, suture is narrower than a 2-0, or 00, suture. The USP assigns the size designations for suture materials giving a precise metric diameter range for the material to be able to achieve a specified tensile strength (1,5). Therefore, among the various suture materials, a larger metric diameter of one material may be necessary to achieve the same tensile strength of another material with a smaller diameter. Both sutures, despite their slight metric size difference, will earn the same USP size designation because both fall within the diameter range set forth by the USP and both achieve the necessary tensile strength (1,5). For example, among absorbable sutures, 4-0 collagen has a larger metric diameter than 4-0 polyglactin 910. Both sutures achieve the same tensile strength and both metric diameters fall within the range necessary to be assigned the 4-0 size designation by the USP (5). The higher the USP size designation number, the smaller the suture diameter, and the less tensile strength a suture will have.
with no. 1 being the smallest diameter. Newer materials and manufacturing techniques then allowed for the creation of smaller-diameter sutures. First, size 0 was accomplished; thereafter, more 0’s were tacked on to the size designation to indicate ever decreasing size diameters. The smaller the suture diameter, the more 0’s are added to the size designation. For example, a 5-0, or 00000, suture is narrower than a 2-0, or 00, suture. The USP assigns the size designations for suture materials giving a precise metric diameter range for the material to be able to achieve a specified tensile strength (1,5). Therefore, among the various suture materials, a larger metric diameter of one material may be necessary to achieve the same tensile strength of another material with a smaller diameter. Both sutures, despite their slight metric size difference, will earn the same USP size designation because both fall within the diameter range set forth by the USP and both achieve the necessary tensile strength (1,5). For example, among absorbable sutures, 4-0 collagen has a larger metric diameter than 4-0 polyglactin 910. Both sutures achieve the same tensile strength and both metric diameters fall within the range necessary to be assigned the 4-0 size designation by the USP (5). The higher the USP size designation number, the smaller the suture diameter, and the less tensile strength a suture will have.
TABLE 4.4 Relative Straight Pull Tensile Strength among Nonabsorbable Suture Materials | ||||||||||
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Stiffness
Stiffness is a suture’s ability to resist stretching. This is important when tissue margins need to be closely approximated and maintained against, for example, tension caused by postoperative edema or muscle contracture (2). If stiffness is too less, soft tissues may gap, correction may be compromised (2). This may lead to dehiscence and wider scar formation when involving the skin. If stiffness is too great, however, an unforgiving suture material may cause damage to the expanding or tensioned soft tissues that it is holding together. Polyglactin 910, PGA, polyester, and steel are examples of materials with a high degree of stiffness (1).
Elasticity and Plasticity
The strength of a suture can be defined as its ability to withstand an applied force before it fails (2). When both suture strength and stiffness are considered, a value of the suture’s elasticity can be determined by dividing the value for stiffness by the value for strength (2). Elasticity is defined as a suture’s ability to regain its original form and length after being stretched (1). Several synthetic suture materials, like nylon, are elastic. These materials stretch with the expanding tissues to a point but are always working to achieve their original shape and size. Plasticity is the suture’s ability to stretch without breaking and retain its stretched, deformed length once the deforming force is removed (1). If the suture material has a high degree of plasticity, it will retain a higher percentage of its original tensile strength when stretched, prior to its failure. It will also maintain this stretched-out length, even after the deforming force subsides, thus limiting the suture loops holding power across the wound margins (1). Polypropylene, a synthetic, nonabsorbable, monofilament suture is an example of a material with a high degree of plasticity (1,15). Polybutester is a synthetic, nonabsorbable, monofilament suture that has a high degree of both elasticity and plasticity (1).
In practice, these properties become especially important when considering postoperative edema (1). As edema causes the expanse of soft tissues, a suture that has elasticity and plasticity will stretch with the expanding tissues (1). If a suture material is too stiff, as soft tissues expand, the suture will resist deformation and cut into and possibly through the tissue, a process referred to as “cutting out” (1,16). Cutting out occurs more frequently with sutures that have high tensile strength and those that are inelastic such as polyglactin 910, PGA, polyester, and steel (1). Cutting out can also occur as a result of tensioning the suture in excess when tying a knot (1). Smaller-diameter sutures also have an increased propensity for cutting out. This is because there is a larger force generated per unit area at the soft tissue-suture interface (1,16,17). Placing the suture too close to the wound margins, not taking a wide enough bite across the incision line, can also make it easier for suture to cut out (16). Dudly showed that the mean force per unit area at the soft tissue-suture interface is reduced as the radius of the suture loop increases (16,18).
Memory is defined as the suture’s tendency to return to its original shape after deformation. It is related to the material’s elasticity, plasticity, and diameter (1). A suture with a high degree of memory will, for example, tend to curl back to its coiled, packaged configuration after it is removed from the package. Suture materials with high memory tend to be stiff and difficult to handle (1). They tie into a less secure knot that can slip and untie as the suture works to achieve its former shape (1). Sutures with high memory require more knot throws and longer cut ends beyond the knot to ensure security (1). Nylon is an example of a suture with high memory (1). Silk is an example of a suture material with low memory (1).
Pliability is subjective term that relates to how easily a suture can be bent. Pliable materials, like silk, are easier to handle and tie more securely into a knot than stiffer materials like monofilament nylon.
KNOT SECURITY
Knot strength is defined as the mean tensile strength of the knotted suture (15). The knot is typically the weakest point in a suture loop (15). Its efficacy is determined by comparing the tensile strength of knotted versus unknotted suture of the same material and same dimensions (15). Knot failure may occur either due to slipping or due to knot break. There are several factors that impact the security of a knot. A knot is deemed secure if it can hold, without significant slipping, to the point of knot break when subjected to a sufficient force (19). When suture breaks, the knot is usually the site of the break and is a result of shear forces generated within the knotted configuration (19). For absorbable sutures, knot strength decreases as the suture is absorbed (19).
Knot failure is more likely to occur due to knot slippage rather than an outright break. Knot slippage is an untying of the knot that occurs, to some degree, with all suture materials (20). This can result in either complete failure of the suture loop or widening of its diameter with resultant separation of soft tissue margins (20). Improper tying technique, the type of knot thrown, and the smoothness of the suture material can all contribute to slippage.
Coefficient of friction is a measure of the slipperiness of a suture material (1). A suture material with a low coefficient of friction is smooth, will glide easily through soft tissues, both
when inserted and removed, with less effort and with less patient discomfort (1). A suture material with a low coefficient of friction will tie easily and glide smoothly into a knotted configuration (1). Once tied, however, the knot may also slip more easily, loosen, or become completely untied (1). Some suture materials, like polyester, are coated with special polymers to decrease their coefficient of friction (1). These coatings improve handling characteristics while maintaining knot security.
when inserted and removed, with less effort and with less patient discomfort (1). A suture material with a low coefficient of friction will tie easily and glide smoothly into a knotted configuration (1). Once tied, however, the knot may also slip more easily, loosen, or become completely untied (1). Some suture materials, like polyester, are coated with special polymers to decrease their coefficient of friction (1). These coatings improve handling characteristics while maintaining knot security.
Suture materials with a high coefficient of friction tend to be rougher in texture. Setting a knot requires more effort as the rough, high-friction material is dragged across itself while being moved into a knotted configuration (1). This same friction, however, will help to maintain the suture in its knotted configuration, even when acted upon by additional forces such as those brought on by edema (1). Plain catgut and monofilament and multifilament steel have high coefficients of friction and have the highest knot efficacy among all suture materials, independent of the type of knot thrown (13). Multifilament, uncoated suture materials such as polyester also have a very high coefficient of friction and rank just below steel and catgut in terms of knot efficacy (Table 4.5).
The type of knot being tied causes more variation in knot efficacy than variations in the type or size of the suture material (13). Simple crossed knots are very inefficient for most suture materials, with steel and catgut being the exceptions (13). For catgut and multifilament and monofilament steel, knot strength has been shown to be independent of the type of knot thrown (13). Most materials, however, require a more complex knot, meaning a greater number of throws, to afford them security. Complex knots, three throws or better, have, on average, greater knot efficacy, almost twice that of simple knots (13,19,21). In some cases, the addition of one extra throw may increase the knot strength by threefold (21). Additional throws are recommended for materials with a low coefficient of friction, monofilament synthetic nonabsorbable and coated sutures (6). Throws added in addition to what will provide for a secure knot do not contribute significantly to the strength of the knot but instead will increase the suture volume in or near the wound. The increased volume offers more surface area to harbor bacteria and amplifies the local inflammatory reaction to the suture (6,22). A balance must be maintained between adding too many throws, too much suture material, and throwing too few loops resulting in a week knot that will slip (21). The square knot and the surgeon’s knot are the most secure and reliable knots for tying most suture materials (6). The surgeon’s knot with the addition of a square knot is very reliable (Fig. 4.1). The number of throws and type of knot needed to make the knot secure varies with different suture materials, and there are often specific recommendations per the manufacture provided in the package insert.
TABLE 4.5 Relative Coefficient of Friction among Nonabsorbable Suture Materials | ||||||||||
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 Figure 4.1 Surgeon’s knot with superimposed square knot. This knot is very reliable for tying most suture materials. Additional throws are recommended for materials with a low coefficient of friction, monofilament synthetic nonabsorbable and coated sutures. |
Knot security is more dependent on the type of material the suture is made of than on the dimensions of the suture (13,23). No clear correlation can be made between thread diameter and knot efficacy for most suture materials (11,23,24 and 25). For some stiffer, monofilament suture materials, such as monofilament nylon, knot efficacy may increase somewhat with narrower-diameter sutures. This is likely due to the more pliable (bendable) nature of the narrower product, which allows the knot to be set more securely (22).
The length of the cut ends of a suture must be long enough to allow the knot to slip to some degree without completely untying (20). Cutting the suture ends too long, on the other hand, leaves more suture material in or near the wound, which may incite an inflammatory response and harbor bacteria (20). It is recommend that a 3-mm tail remain to offer some room for knot slip while minimizing the amount of extra suture left in or near the wound. Some materials are more prone to knot slip. These materials tend to have a high degree of memory and a low coefficient of friction, as seen with some synthetic monofilaments like nylon. These materials require additional throws and longer cut ends, up to a 6-mm tail, to guard against knot failure due to slippage (6).
Surgeon’s technique is a major factor in knot security. It has been demonstrated that improper knot tying technique can lead to knot slippage and failure (22). The knot must be secured snugly into place with uniform tension maintained on both ends of the suture to insure a square knot is being tied and not a slip knot, which is relatively insecure (21,24).
BIOLOGIC PROPERTIES
All suture materials, both absorbable and nonabsorbable, are recognized as a foreign body and will elicit some degree of a soft tissue response (1,2,11). The degree and severity of the response will vary depending on the type of material, its configuration, and the quantity of material implanted (1,26).
Several simultaneous histologic reactions occur as the host tissue reacts to the foreign material. Cells are recruited to clear debris, break up, and absorb suture material. At the
same time, other cells are recruited to make collagen to heal the wound (11,12).
same time, other cells are recruited to make collagen to heal the wound (11,12).
The initial soft tissue reaction, which is the same for all types of needles and all types of suture materials, is the result of the inherent damage sustained during the passage of the suture material and needle (1,12,27,28). The cellular response to the implanted suture material then begins and will peak at 2 to 7 days postimplantation (1,29). After this initial response, between 7 and 10 days postimplantation, variations in the inflammatory reaction can then be seen for different types of suture (12,30).
Minimizing the soft tissue reaction is important because a prolonged and severe reaction may lead to a heavy buildup of exudates and debris, which may delay wound healing and increase the risk of infection (1,2,27,31,32 and 33). A large inflammatory reaction zone around the implanted suture material can also affect how well the suture is able to hold the tissue margins in apposition (31,32,34). A wide reaction zone means the tissue inside the suture loop may be weakened, thereby increasing the risk of the suture material cutting out of the wound margins (31). Madsen (30,31 and 32) showed that plain catgut caused a severe soft tissue reaction with large amounts of exudates and a wide reaction zone around the suture. This caused a delay in collagen formation and a delay in wound healing (30,31 and 32). If wound healing is delayed, the risk for wound dehiscence increases as the weakened incision site may lack sufficient strength to support itself by the time the suture is either removed or absorbed (30,31 and 32).
The degree and severity of soft tissue response to absorbable sutures varies among the materials but is overall greater than that elicited by nonabsorbable sutures. Multifilament materials and natural materials will elicit more of a response than monofilament and synthetic materials (1,34). Plain catgut elicits the most severe inflammatory response among all suture materials followed by chromic catgut then natural fiber materials such as silk (24,30,31 and 32,34). For all suture materials, the inflammatory reaction will be greater with an increase in the volume of implanted suture (1,22,31,32).
NATURAL ABSORBABLE SUTURES
Surgical Gut
Natural or gut sutures are interwoven collagen fibers that are formed into a single strand. It is available in plain or chromic forms, the later being treated with chromium salts to delay the soft tissue reaction and slow its degradation. A decreased soft tissue response allows for relatively milder exudates and a decrease in the rate at which it loses its tensile strength (33). The term catgut may be derived from kitgut, which are narrow strings of a violin made of animal intestine (1). Catgut, or surgical gut as it is called, can be made from the intestinal submucosa of sheep or the intestinal serosa of cattle (1,6,12). The collagen in these sutures is protein and elicits a rather severe antigenic response once implanted. These sutures are broken down in the body by enzymatic digestion and phagocytosis (1,28). The severity of the tissue response to these products may vary depending on their purity and the concentration of chromium salts applied during the manufacturing process. Early manufacturing of collagen-based sutures resulted in unreliable purity with contaminants like muscle or mucoproteins incorporated within the suture filaments. These contaminants cause an even greater inflammatory reaction and weaken the suture (1,6). Contemporary manufacturing techniques offer a more pure and standardized collagen suture with more predictable properties (6,35).
Degradation of catgut begins at 12 hours postimplantation and is carried out by lysosomal proteolytic enzymes (1,28). Degradation peaks at 3 days postimplantation, and debris from the breakdown is removed via giant cells and other phagocytes beginning 7 to 10 days postimplantation (1,28). It is not until absorption of the suture begins that the soft tissue reaction becomes severe (28). Chromic catgut elicits a relatively mild initial soft tissue reaction during its first 10 days of implantation, similar to that elicited by synthetic nonabsorbable sutures (30,31,32,33 and 34). These 10 days give the soft tissues an opportunity to heal sufficiently (33). As the chromium salts are broken down and absorbed, the soft tissue response becomes more marked (30,31,32,33 and 34). Chromic salts added to catgut may elicit an allergic reaction in patients who are chromate sensitive (1).
For both plain and chromic catgut, especially plain catgut, there is a relatively large soft tissue reaction at 10 to 15 days postimplant with large numbers of histiocytes, fibroblasts, leukocytes, and giant cells (12). During this time, cellular ingrowth and absorption of the suture filaments are seen (12). Because of this intense response, plain catgut should not be used for cutaneous suturing as it may interfere with wound healing and increase wound susceptibility to bacterial infection (1,2,26,33).
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